The present invention generally relates to a method of inspecting or examining a surface of a specimen as to the presence or absence of imperfections such as defects, damages, injuries, scratches or the like and/or foreign substances such as contaminants or the like. In particular, the method according to the invention is intended to be applied to such inspection or examination of a blank or substrate material for a magnetic disc on which undesired fine topographical marks such as cutting marks are formed with a certain directivity (i.e. in a predetermined direction). The present invention also concerns an apparatus for carrying out the above-mentioned method.
For having a better understanding of the invention, hitherto known methods and apparatus for detecting the imperfections or defects on specimens such as blank materials for magnetic discs will be described in some detail. FIG. 1 of the accompanying drawings shows a typical one of the conventional inspecting apparatus of a scattered light collect-type. Referring to the figure, a laser light beam 1a is generated by a laser oscillator 1 and directed to a surface of a specimen 4 for irradiation thereof through a mirror 2 and a lens 3. When the surface of the specimen 4 to be examined is perfectly flat or smooth, all of the laser light 1a undergoes normal reflection at the specimen surface and follows its incident path in reverse to the oscillator through a center hole formed in a mirror 5. When the surface of the specimen 4 is roughened, incident laser light is scattered as indicated by broken lines 1b with arrows and collected by the lens 3 to be directed to a photoelectric tube 6 after having been deflected by the mirror 5 having the center through-hole. When a surface of a metallic specimen 4 having cutting marks is in concern, the laser light undergoes such irregular reflection that the light is scattered in the direction (x-direction) which extends orthogonal to the cutting marks 4a provided on the surface in the form of fine grooves or recesses extending in the y-direction, as indicated by arrow-headed broken lines 1c in FIG. 2. FIG. 3 shows in a polar coordinate a distribution of intensity of light having undergone such irregular reflection. When the cutting marks are formed through grinding or the like machining, components of the irregularly scattered light rays in the x-direction lie predominantly within an angular range .theta. of about 1.degree.. In other words, the irregularly reflected or scattered light exhibits a significant directivity. It is now assumed that an isolated or discrete point-like defect 4b such as a depression or a dimple is present on the surface of the specimen 4, as is illustrated in FIG. 4. When the diameter of the irradiating laser beam is larger than that of the isolated point-like defect 4b, there is obtained an intensity distribution 7b of the irregularly reflected or scattered light, which distribution exhibits a lesser directivity, as shown in a polar coordinate in FIG. 5. The light flux 1c impinging on the photoelectric tube 6 thus includes the irregularly scattered light reflected from the cutting mark and the scattered light rays reflected from the isolated point-like defect 4b, resulting in that an output voltage signal derived from the photoelectric tube 6 has a signal waveform such as illustrated in FIG. 6. In the case where a linear defect 4c such as scratch is present on the surface of the specimen 4 as illustrated in FIG. 7, then there is produced an intensity distribution 7b of the light irregularly reflected from the linear defect 4c, which distribution exhibits a strong directivity in the direction perpendicular to the longitudinal direction of the scratch 4c, as is illustrated in a polar coordinate in FIG. 8. In this case, the voltage signal output from the photoelectric tube 6 has a waveform similar to the one illustrated in FIG. 6.
It will be appreciated that the voltage output from the photoelectric tube 6 obtained with the defect detection system described above undergoes inevitably an offset which corresponds to the intensity of the irregularly reflected light scattered by the cutting marks, involving extremely lowered detection sensitivity for the defects such as the isolated point-like defect 4b and the linear defect or scratch 4c, to a disadvantage. This is ascribable to such sensitivity characteristic of the photoelectric tube 6 that the output voltage thereof is saturated when the incident light is bright, while exhibiting a linearity in the sensitivity when the incident light is in a lower intensity range. By way of example, when a ground surface (0.05 .mu.m R.M.S.) of an aluminium plate is inspected by irradiating with a laser beam of 50 .mu.m in diameter through a lens of 0.4 in numerical aperture, the possibility of detecting the defect is limited to the point where the diameter and width of the defect is about 10 .mu.m and 1 .mu.m, respectively.